Following infection with the protozoan parasite Leishmania major, C57BL/6 mice develop a small lesion that heals spontaneously. Resistance to infection is associated with the development of CD4⁺ Th1 cells producing gamma interferon (IFN-γ) and tumor necrosis factor (TNF), which synergize in activating macrophages to their microbicidal state. We show here that C57BL/6 mice lacking both TNF and Fas ligand (FasL) (gld TNF^−/− mice) infected with L. major neither resolved their lesions nor controlled Leishmania replication despite the development of a strong Th1 response. Comparable inducible nitric oxide synthase (iNOS) activities were detected in lesions of TNF^−/−, gld TNF^−/−, and gld mice, but only gld and gld TNF^−/− mice failed to control parasite replication. Parasite numbers were high in gld mice and even more elevated in gld TNF^−/− mice, suggesting that, in addition to iNOS, the Fas/FasL pathway is required for successful control of parasite replication and that TNF contributes only a small part to this process. Furthermore, FasL was shown to synergize with IFN-γ for the induction of leishmanicidal activity within macrophages infected with L. major in vitro. Interestingly, TNF^−/− mice maintained large lesion size throughout infection, despite being able to largely control parasite numbers. Thus, IFN-γ, FasL, and iNOS appear to be essential for the complete control of parasite replication, while the contribution of TNF is more important in controlling inflammation at the site of parasite inoculation.